Of the Americans Robert Mueller has indicted in his rolling inquest into Russian subversion of the Trump campaign—George Papadopoulos, Rick Gates, Mike Flynn—only Donald Trump’s former campaign chairman, Paul Manafort, has resisted cooperating with the special counsel, pleading not guilty to nearly two dozen charges of conspiracy, money laundering, and tax and bank fraud charges stemming from his murky lobbying work in Ukraine. But the legal calculus facing Manafort appears to be changing rapidly. Earlier this week, the president declared him persona non grata, withdrawing the prospect of the presidential pardon Manafort might have been waiting for. Most significantly, on Monday the special counsel’s office accused Manafort of witness tampering—a crime considered to be a violation of his bail terms that could land him in jail until the completion of his two trials, scheduled to begin in July and September.

The details of the alleged scheme are spectacular in their brazenness. According to court documents filed with a Washington D.C. district court on Monday, Manafort attempted to contact members of a firm informally known as the “Hapsburg” group, which he used to advocate for the Party of Regions, a pro-Russian political party in Ukraine. As detailed in earlier court filings, Manafort funneled more than 2 million euros between 2012 and 2014 through the firm. On Monday, Mueller alleged that Manafort and an individual identified as “Person A”—who fits the description of his longtime business associate Konstantin Kilimnik—tried to contact two witnesses in the case “to secure materially false testimony” via telephone and two encrypted messaging apps, WhatsApp and Telegram, about the activities of the Hapsburg group. Mueller—who was able to confirm the messages, as they were backed up to iCloud—argues that Manafort sought to get the two witnesses to testify that the lobbyist group only worked in Europe, not in the United States, reflecting a critical piece of Manafort’s defense. One of the witnesses, identified as Person D1, told F.B.I. agent Brock Domin that they understood “Manafort’s outreach to be an effort to ‘suborn perjury.’”

The allegations, if true, suggest a startling naïveté for a longtime political operator like Manafort. A list of telephone calls and text messages collected by Mueller’s team show that Manafort was under observation all along—a fact that might have occurred to him while under house arrest, wearing not one but two G.P.S.-enabled ankle monitors. Apparent attempts by Manafort’s associates to disguise his identity (“My friend P”) were also deeply unsophisticated. Nor is this the first time Mueller has caught Manafort engaging in improper activities following his indictment. Shortly after Manafort was released on bail earlier this year, Mueller’s team intercepted e-mails that indicated the former Trump campaign chairman had aided in the writing of an opinion piece defending his work for the Party of Regions and former Ukrainian president Viktor Yanukovych.

This latest dustup, however, could land Manafort in a jail cell. Mueller is alleging that Manafort’s witness tampering amounts to a federal crime in direct violation of his house-arrest conditions. “Manafort’s efforts to influence the testimony of potential witnesses, both directly and through an intermediary, threaten ‘the integrity of the trial’ even though those efforts have not been violent in nature and the underlying prosecution involves ‘white-collar’ offenses rather than ‘narcotics’ or violent crimes,” the filing reads. A federal judge has given Manafort until Friday to respond to Mueller’s accusations of witness tampering, The Washington Postreported. In a statement on Tuesday, Manafort spokesman Jason Maloni said, “Mr. Manafort is innocent, and nothing about this latest allegation changes our defense. We will do our talking in court.”

Manafort’s actions, while perplexing from the outside, may help to shed light on his thinking. For months, legal experts have scratched their heads at Manafort’s motives in fighting Mueller’s case, which appears to be damning. Sources that have spoken to my colleague Chris Smith have offered various explanations for Manafort’s strenuous assertions of innocence: that he is more scared of the people he might cross than he is of Mueller; that he is laying the groundwork to paint the prosecution as biased, riding on Trump’s coattails; that he believes his information is so good that he can cut a deal later; or perhaps that the evidence against him is weaker than it seems. Monday’s filing, however, suggests a different motive. If Manafort was pressuring witnesses to present false testimony in the case, it may indicate that Manafort is indeed guilty, is aware of his guilt, and is actively trying to suborn perjury to secure a not-guilty verdict. In short, it looks like Manafort thought he could win his case by playing dirty.

Now, with Mueller’s latest filing, the calculus has changed—for Manafort, the special counsel, and for Trump. If Manafort does decide to cooperate, he may have much to say. While he served as Trump’s campaign chairman for less than six months—a fact the White House has repeatedly highlighted—Manafort was at the helm when Trump secured the Republican Party nomination and during episodes Mueller is probing, including the June 2016 Trump Tower meeting. Of anyone in Trump’s orbit, Manafort also boasted the deepest ties to the Russian government dating back to his lobbying work in Eastern Europe. From the Monday filing, it is clear that Mueller has already put to use information gleaned from Gates, Manafort’s longtime business associate, who served as his deputy on the Trump campaign and who pleaded guilty earlier this year. And of the two, Manafort is the bigger fish.

Could Manafort’s stumble inspire Trump to reconsider his fidelity? We know that in October of last year, Trump’s then-lawyer John Dowdreportedly floated the possibility of a presidential pardon to Manafort’s lawyer. Since Mueller indicted Manafort, however, Trump and the White House have sought distance from the disgraced political operative. In a series of two tweets earlier this month, the president wrote that the F.B.I. should have informed him of the investigation into Manafort, and said that if he had known about Manafort’s background, he never would have hired him. At the same time, Manafort is now in a more precarious position than ever—a fact that makes him deeply dangerous to the president, should he decide upon a more conciliatory legal strategy. At the end of last month, D.C. District Court Judge Amy Berman Jackson rejected Manafort’s bid to have the charges against him dismissed, in which he argued that they fell outside Mueller’s mandate—representing a major blow to his case. Manafort could fare better in a Virginia court, where Judge T.S. Ellis III has not yet made a ruling on whether the charges should be dismissed. But if that verdict also comes down against him, Manafort’s best option could be cutting a deal with the special counsel. With the court filing on Monday, Mueller sent a very clear message: things can only get worse.